wow, this is great news from Sennheiser! Finally the speculation is put to rest.
So, we either have two cases. Either (1) the HD800 we were expecitng is going to be called the HD680, or (2) they are two unique headphones.
If case 1 turns out to be true, I expect MSRP to be 700-900$us if they keep the HD600 and HD650. If they discontinue either the HD600 or HD650, I expect MSRP to be 600-750$.
If case 2 turns out to be true, I espect MSRP to be 600-750$, with the HD800's MSRP ~1000-1200$us.
Either case, the HD680 will not hit street price for a few months at the very least. I think within a month or so we can expect it to be 50-100$ off MSRP.
Either way, if you want one right when they come out, it's going to be pricey.